(1) Field of the Invention
This invention relates to a highly electroconductive graphite continuous filament and a process for preparing the same.
(2) Description of the Prior Art
Electrically conductive metals such as metallic copper and aluminum have heretofore been used as electrically conductive materials. However, resources of these metals will be exhausted at some time or other, and development of electroconductive materials that can be used as substitutes for these metals has been desired. Furthermore, since metals have a large specific gravity, in the field where the light weight characteristic is required, development of a light electroconductive material is desired. Moreover, metals are corrosive, and hence, the application fields are limited. Accordingly, development of anti-corrosive electroconductive materials has long been desired. Still further, since metal conductors have a relatively low melting point, they cannot be used at a very high temperature. Therefore, development of electroconductive materials that can be used at a super-high temperature has been desired. An electroconductive material satisfying these requirements should have an electric conductivity of at least 1.0.times.10.sup.4 S/cm, preferably at least 5.0.times.10.sup.4 S/cm, should be flexible, stable, light and anti-corrosive and should resist a high temperature and be in the form of continuous filament.
It is known that graphite has a high electric conductivity. However, graphite is obtained only in the form of a small piece, and is not suitable for use as an electroconductive material.
A carbon fiber has a filamentary shape suitable for industrial purposes, but the electric conductivity is low, i.e., about 6.times.10.sup.2 to about 1.times.10.sup.3 S/cm at 20.degree. C. Even if it is calcined at a temperature higher than 3,000.degree. C., the electric conductivity is about 2.times.10.sup.3 S/cm and the calcined product is still not suitable as an electroconductive material.
It has been reported that a graphite fiber was manufactured according to the gas phase growth method [A. Oberlin, Carbon 14, 133 (1976)]. However, in view of this manufacturing method, the fiber can be obtained only in a short fiber form having a length of about 25 cm at longest, and the electric conductivity is inevitably reduced at joints of fibers and thus, the fiber does not meet the afore-mentioned demands. As means for preparing a carbon-carbon composite, there has been proposed a method in which carbon is deposited on carbon staple fibers or a woven fabric of carbon fibers by CVD (chemical vapor deposition) and the carbon staple fibers or woven fabric is then heat-treated. In the product obtained according to this method, carbon fibers are fusion-bonded to one another and therefore, the product has poor flexibility and cannot be used as an electroconductive material. Moreover, even if calcination is carried out at such a high temperature as about 3,000.degree. C., the electric conductivity of the composition obtained according to this method is as low as about 3.times.10.sup.3 S/cm (see, for example, page 84 of International Symposium on Carbon, Toyohashi, 1982), and the composition is not suitable as an electroconductive material. This fact is very important and indicates that even if carbon deposited by the CVD method is calcined at a high temperature, the electric conductivity is not necessarily highly improved. Japanese Examined Patent Publication No. 41-12,091 proposes a method for preparing a carbon fiber by thermal decomposition of benzene. However, the electric conductivity of the fiber obtained according to this method is as low as about 2.times.10.sup.3 S/cm and the fiber length is about 10 cm at longest.